The invention relates generally to optical communication systems employing wavelength division multiplexing (WDM) and, more particularly, to controlling optical signal power when individual optical channels are added or dropped at nodes in such systems.
Optical fiber is fast becoming a transmission medium of choice for many communication networks because of the speed and bandwidth advantages associated with optical transmission. In addition, wavelength division multiplexing (WDM) is being used to meet the increasing demands for more capacity in optical communication networks. As is well known, WDM combines many optical channels each at a different wavelength for simultaneous transmission as a composite optical signal in a single optical fiber. By using optical transmission and WDM in the backbone networks, the communications industry has made great strides in terms of offering greater capacity and transmission speeds in today""s networks.
Management of this increased capacity in WDM systems, i.e., managing the communications traffic transported in many different optical channels, is an important aspect of any WDM-based communication network. For example, WDM systems typically include an add/drop capability whereby signals transported on the individual optical channels can be selectively added or dropped at various nodes in a network.
One of the challenges associated with adding and dropping optical channels in existing systems is controlling the signal power of a WDM signal at an add/drop node and, in particular, controlling the signal power of the individual optical channels that are added, dropped, or directly routed through the node without being either added or dropped. For example, power divergence is a problem that can occur in which optical channels in a WDM signal have different signal power levels. By way of example, power divergence can occur in an add/drop node because different optical channels are routed along different paths and through different components within the add/drop node. In particular, those optical channels being dropped will be routed through a path containing components for removing the optical channels of interest from the WDM signal. Similarly, optical channels being added originate from and are routed through other components in another transmission path. Finally, optical channels that are capable of being dropped as well as those optical channels that are expressly routed through a node each may traverse a different transmission path within the node. The power levels of each of these types of optical channels can therefore differ because of the different loss characteristics of the components within each of the paths as well as the different compensation schemes (e.g., optical amplification) that may be used within any of the transmission paths.
Another cause of power divergence among the different optical channels of a WDM signal is so-called xe2x80x9cripplexe2x80x9d, which is a well-known phenomenon in optically amplified systems. In particular, a spectrum of optical channels in a WDM signal may accumulate tilt and ripple effects as the WDM signal propagates along a chain of optical amplifiers, e.g., multiple optical repeater nodes spaced between end terminals and add/drop nodes. As is well-known, ripple is manifested as a substantially non-random power divergence whereby signal power across the spectrum of optical channels in a WDM signal varies in a somewhat sinusoidal-type pattern or profile that is sometimes referred to as a xe2x80x9cripple curvexe2x80x9d. Generally, gain flatness is a desirable characteristic of optical transmission whereby the gain is relatively flat across the various wavelengths (i.e., optical channels). It is therefore desirable to compensate for the peak-to-peak deviation, e.g., ripple, of signal power in an optically amplified WDM signal. As such, gain equalization techniques are commonly employed to flatten or tilt a broadband optical amplifier profile to obtain spectral flatness and low ripple in the WDM signal. However, incorporating gain equalization filters to compensate for ripple at the input of every add/drop node would introduce an unacceptable amount of loss. Adding optical amplification to compensate for these additional losses can lead to more noise, e.g., higher noise figure in the optical amplifier and a lower optical signal-to-noise ratio.
Compensating for ripple at an add/drop node is also complicated by other factors. In particular, individual optical channels in a WDM signal are routed along different paths and through different components within the add/drop node depending on whether the optical channel is being dropped, added, or routed through the node either directly or indirectly. As such, ripple in the incoming WDM signal may be carried through the node on certain optical channels but not others, e.g., on the xe2x80x9cexpressxe2x80x9d channels routed through a node and not on the channels being added at the node. The different optical signal power levels for the individual optical channels as well as gain ripple must therefore be taken into consideration when combining the optical channels to generate the WDM output signal from the add/drop node.
The optical signal power of a WDM signal processed at an add/drop node is controlled to account for uncorrected ripple in the WDM signal according to the principles of the invention by adjusting the optical signal power level of optical channels being added at the add/drop node to match the ripple exhibited by optical channels that are expressly routed through the add/drop node. In this manner, the ripple in the WDM signal being output from the add/drop node approximately corresponds to the ripple in the incoming WDM signal at the node.
In one illustrative embodiment, an add/drop node in a WDM system receives a WDM input signal exhibiting ripple. The add/drop node includes an xe2x80x9cexpressxe2x80x9d transmission path for routing selected optical channels of a WDM input signal directly through the node, a xe2x80x9cdropxe2x80x9d transmission path for dropping selected optical channels from the WDM input signal, a xe2x80x9cthroughxe2x80x9d transmission path for routing selected optical channels through the node that are not being dropped, and an xe2x80x9caddxe2x80x9d transmission path for adding selected optical channels. The optical channels from the xe2x80x9cexpressxe2x80x9d, xe2x80x9cthroughxe2x80x9d, and xe2x80x9caddxe2x80x9d transmission paths are combined to form a WDM output signal. According to the principles of the invention, a target signal power level is determined for the optical channels in the xe2x80x9cthroughxe2x80x9d path and the total signal power of the optical channels in the xe2x80x9cexpressxe2x80x9d path is coarsely adjusted to a level that is approximately equal to the target signal power level. The signal power levels of individual optical channels in the xe2x80x9cthroughxe2x80x9d and xe2x80x9caddxe2x80x9d paths are then adjusted on a per-channel basis as a function of the ripple that is present in the optical channels in the xe2x80x9cexpressxe2x80x9d path. In this manner, a xe2x80x9cripple fittingxe2x80x9d adjustment is made so that the signal power of the xe2x80x9caddxe2x80x9d and xe2x80x9cthroughxe2x80x9d optical channels effectively matches or follows the ripple that is present in the incoming WDM signal.